Kadrian Sey was a Zabrak Jedi of the Old Republic operating on the fringe worlds prior to the eruption of the Clone Wars. Following the Battle of Geonosis, Kadrian was one of the many Jedi who did not report to Coruscant at the behest of the Jedi Council. A few months later it was discovered why. At some point, Kadrian Sey fell to the Dark Side and into league with Count Dooku. She joined Tol Skorr, also a fallen Jedi, as one of Dooku's top henchmen, and the two continually vied for favor from Dooku.
When Quinlan Vos began his mission to infiltrate Count Dooku's circle, Kadrian and Skorr united to present somewhat of an initial test for Vos in order to gain the Count's audience. Vos was easily the better warrior than both Sey and Tol Skorr and bested the two in combat. As one of Dooku's minions, Kadrian joined the Separatist leader on many missions, including one to the planet of Tibrin, home of the Ishi-Tib, in order to depose their butcher of a leader, Suribran Tu, and welcome Tibrin into the Confederacy of Independent Systems. Following this, Kadrian was responsible for the interrogation of Jedi Master Shylar, another Jedi who attempted to infiltrate Dooku's camp. Sadly for Jedi Master Shylar, she died due to the injuries she sustained at the cruel hands of Kadrian.
Another mission took Count Dooku and his squad of enforcers to Quinlan Vos' home planet of Kiffu. Dooku hoped to convince Shyef Tinte to allow the Separatists to establish a base on Kiffu. However, Tinte rebuffed and rejected Dooku's request emphasizing that the Kiffar would remain independent during the war and emerge that much stronger once the war was over. Disappointed in Tinte's answer, Dooku ordered Kadrian, Quinlan, Tol to kill Tinte and her Guardians. However, Sheyf Tinte was a relative of Quinlan Vos and when Kadrian Sey moved in for the kill his loyalties to his family as well as the Jedi superseded his mission of infiltration. As a result, Quinlan slashed at Kadrian Sey's torso killing her and saving Sheyf Tinte.